1. Field of the Invention
This invention relates to the condensation of metal vapor, and more particularly to a lead-splash condenser for condensing metal vapor, especially zinc vapor, from a hot gas stream.
2. Description of the Prior Art
A pyrometallurgical zinc smelting process is known in which oxidic zinc/lead material is reduced with coke in the shaft of a blast furnace. A hot gas stream, containing zinc vapor, passes from the top of the furnace shaft to a multi-stage lead-splash condenser, wherein the zinc vapor is condensed in a plurality of condensation chambers by a spray of molten lead droplets. Molten lead containing dissolved zinc is withdrawn from the condenser and cooled to effect a separation of molten zinc, and cooled de-zinced molten lead is recirculated to the condenser. Such a smelting process is more fully described in "Application of the Blast Furnace to Zinc Smelting" by Morgan & Woods, Metallurgical Review 16, November 1971.
The gas leaving the condenser, from which most of the zinc vapor has been condensed, is scrubbed to remove fume and dust carried over from the furnace shaft. Large quantities of leady, mainly oxidic, material known as "blue powder" are recovered and returned to the furnace shaft for re-smelting. The production of blue powder, mainly caused by the oxidation of lead and zinc emission from the furnace and condenser, is undesirable for a number of reasons.
The recycled load of blue powder to the furnace shaft reduces the capacity of the furnace to smelt new metal; additional coke has to be burnt to recover metal values, and the condenser gas offtake ducts become severely accreted after a period of operation.
The blue powder contains about 30% by weight lead and 30% by weight zinc, the balance consisting of oxides and sulphides of iron and other metalliferous materials. The zinc content arises from the oxidation of unabsorbed zinc vapor leaving the condenser, while the lead content arises partly from the volatilization of lead sulphide in the furnace shaft and partly from the elutriation of lead droplets in the gas stream leaving the condenser.
In a conventional lead-splash condenser the gas outlet duct from the condenser is reached via a substantially vertical stack. We have now found that the gas flows in a stream up the stack along the side wall thereof which faces the outlet from the last condensation chamber, i.e. along the end wall of the condenser, at a relatively high speed. Typically, the speed of the gas stream along this side wall of the stack may be about four times the theoretical speed which would prevail under ideal conditions of uniform gas flow through the stack. The relatively high streaming velocity of the gas tends to increase the rate of droplet elutriation, and it is an aim of the present invention to improve the gas flow distribution pattern in the stack and to thereby reduce the rate of elutriation of lead droplets in the gas stream leaving the condenser.